1. Field of the Invention
This invention relates to a porous hollow fiber membrane and a method for the removal of a virus by using the same. More particularly, the present invention is concerned with a novel porous hollow fiber membrane which is characterized by its unique porous structure wherein the inner and outer membrane surfaces have an in-a-plane average pore diameter of 0.01 to 10 .mu.m and the porous membrane wall has an in-a-plane porosity of not less than 10% measured in every plane perpendicular to a radial direction of the annular cross-section of the hollow fiber membrane, said in-a-plane porosity exhibiting at least one minimum value between the inner and outer membrane surfaces. The present invention is also concerned with a method for the removal of a virus from an aqueous protein solution containing a virus by the use of the above-mentioned porous hollow fiber membrane. The novel hollow fiber membrane and the method of the present invention are especially useful because they are extremely effective for the removal of a virus with the great advantages that both an excellent virus removal percentage and a high filtration speed can be simultaneously attained.
2. Discussion of Related Art
Methods for the removal of viruses from aqueous solutions by using a uniform and symmetrical membrane (e.g. the microporous polyethylene hollow fiber disclosed in U.S. Pat. No. 4,401,567) are disclosed in Japanese Patent Application Laid-Open Specification Nos. 60-142860, 60-142861 and 61-168367. In these methods of prior art, hollow fiber membranes having an effective thickness of 5 .mu.m or more and a uniform pore structure are utilized to remove viruses. As an example of such hollow fiber membranes, there can be mentioned a polyethylene microporous hollow fiber which has rectangular pores formed by microfibrils that are oriented in the lengthwise direction of the fiber and knotted portions that are connected to said microfibrils substantially at right angles thereto, the average width of the pores being in the range of from 0.05 to 0.35 .mu.m, the pores being contiguous with each other from the inner wall surface to the outer wall surface to form a stacked, multicellular structure. In the specifications of these Japanese patent applications, there is a description to the effect that when the filtrate obtained by the filtration of HBs antigen-positive fresh human plasma using the above-mentioned hollow fiber membrane was observed by an electron microscopy, there was detected no Dane particle having a diameter of 0.042 .mu.m. In this connection, however, it should be noted that there is no description with respect to the actual virus removal percentage (the measurable upper limit of virus removal percentage by electron microscopy is about 99%). Further, since in these cases the transmembrane pressure which is one of the filtration conditions is 50mmHg or less, and the filtration speed is extremely low, such a filtration method cannot be commercially employed for removing viruses. Furthermore, since the filtration speed is low, the physiological activity of the filtrate becomes extremely low.
On the other hand, in the Japanese Patent Application Laid-Open Specification No. 61-254202, a method is disclosed for the removal of tabaco mosaic virus from an aqueous solution containing ovalbumin by using a porous hollow fiber made of cuprammonium regenerated cellulose which has an average pore diameter of 0.02 to 0.2 .mu.m and an in-a-plane porosity of 10% or more. However, with this method, the virus removal percentage is about 99% and the ovalbumin permeability is 43%, which is insufficient for practical use. This hollow fiber has a relatively uniform pore structure.
With the above-mentioned conventional methods in which an asymmetrical, or uniform, symmetrical porous membrane is employed, high virus removal percentage and high filtration speed (or high permeability for protein) cannot be simultaneously attained. The protein permeability of the conventional porous membranes is about 50%.
In general, if the average pore diameter of a porous membrane is decreased, the virus removal percentage is increased but the filtration speed and the protein concentration of the filtrate are lowered. If the average pore diameter is increased, the virus removal percentage is lowered to 99% or less, which is insufficient for a membrane to be used for the removal of viruses. The virus removal percentage normally required for a virus removing membrane is as high as 99.99 to 99.999999%. Thus, there has been a technical dilemma that a porous membrane cannot be simultaneously characterized by an excellent virus removal percentage and a high filtration speed when it is used for the removal of a virus from an aqueous protein solution containing a virus. Therefore, it has been desired to solve the dilemma and develop a porous hollow fiber membrane having both an excellent virus removal percentage and a high filtration speed.